This invention relates in general to membranes suitable for use as dialysis membranes and, more particularly, to a unique process to fabricate dialysis membranes.
Dialysis membranes are used in a variety of applications to separate particular constituents from liquids. For example, such membranes have been used to remove salt from seawater, treat industrial waste, and to purify chemicals.
One use for dialysis membranes is in the removal of waste chemicals from electrodeposition processes. An electrodeposition process utilizes a tank or bath filled with an electrically-charged paint or coating. An electrically-conductive object to be coated is placed into the tank along with a separate electrode device and a source of electrical potential (voltage) is used to create a voltage difference between the object and the electrode. The charge on the coating causes it to be attracted to the object when the object is charged by the electrical potential, much like static electricity acts to pull articles of clothing together. A dialysis membrane is used as part of the electrode device to separate the electrode from the solution within the electrodeposition tank. A variety of such electrode devices are known, including those shown in U.S. Pat. Nos. 5,049,253 and 5,507,929, which are hereby incorporated herein by reference. As is shown in these patents, in these electrode devices the electrode is separated from the electrocoating solution by a dialysis membrane that generally surrounds the electrode. A space is provided between the membrane and the electrode for accumulation of neutralized waste which migrates through the membrane. Neutralized waste that passes through the membrane barrier is flushed from the area between the electrode and the membrane and removed from the process. Because of the configuration required for such electrodes, the membrane utilized needs to be generally self-supporting, non-fouling, easily cleaned and sealable between the electrode body and the membrane.
Dialysis membranes currently are produced utilizing a variety of methods, each with its own set of advantages and disadvantages. Some typical ways of manufacturing such membranes are described in U.S. Pat. No. 5,049,253, noted above. One method of producing a dialysis membrane involves impregnating a tubular polyethylene porous body or substrate with a monomer mixture liquid comprising a polymerizable monomer having a functional group suitable for introduction of an ion exchange group, which is then cross linked. Subsequently, an ion exchange group is introduced to create a working membrane. The ion exchange groups are thus deposited into the voids present in the porous body. One limitation of this method is that the effectiveness of the membrane is curbed by the relative proportion of ion exchange groups which can be deposited by this process. First, voids must be present in the substrate which can be filled by the ion exchange groups. Because of difficulties inherent in producing a stable substrate with a distribution of small voids, the relative number of voids is constrained. Second, the inefficiency inherent in depositing ion exchange media into the voids in the substrate further limits the portion of the ion exchange media present in the finished membrane. Thus, media formed by this method tend to be relatively inefficient. Moreover, in the process of making the substrate with voids and, subsequently, filling a portion of the voids with ion exchange media, the surfaces of the resulting membrane thus formed are relatively rough. In use, this roughness tends to allow collection of particles which produce fouling and further reductions in the efficiency of the membrane. For example, such fouling occurs in electrodeposition process due to collection of neutralized components and other accumulated debris resulting from the process coating the membrane. Surface roughness also can cause problems in sealing the membrane to the electrode or similar device. Moreover, the exterior roughness inhibits cleaning of the membrane and, ultimately, the useful life of the product. In addition, the porous membrane tends to swell and deform in use, creating sealing problems and leading to leaking and failures of the membranes in operation.
Another method of producing a dialysis membrane entails mixing ion exchange groups as a resin powder and a heat-moldable thermoplastic resin, such as an olefin resin, together in dry form. The mixed components are then extrusion molded into the requisite shape, typically a tubular shape. This extrusion method resolves to some extent the fouling problem associated with the previously described deposition process, because the extruded surfaces are generally smooth, and, also, resolves some of the limitations on the amount of ion exchange material which may be placed in a porous substrate as discussed above. However, the amount of olefinic resin required to make a mechanically resistant product reduces the amount of ion exchange material that can be used in the extrusion process. Also, the extrusion process stratifies the distribution of the ion exchange media in the finished dialysis membrane. This stratification limits the amount of ion exchange component which can take part in dialysis reactions, making the membrane more resistive to ion propagation. Thus, although a greater proportion of the ion exchange media is deposited in the membrane, this stratification inhibits the effectiveness and efficiency of the finished membrane. Moreover, the extrusion process limits the available membrane shapes to those commonly achievable from an extrusion process, and effectively prevents specialized shapes, such as creating textures on the ends of membranes in order to facilitate sealing.